The oceans mediate the response of global climate to natural and anthropogenic forcing. Yet for the past 2000 years, a key interval for understanding the climate response to these forcings, global sea surface temperature changes and the underlying driving mechanisms are poorly constrained. Here we present a global synthesis of sea surface temperatures for the Common Era (CE) derived from 57 individual marine records that meet strict quality control criteria. We observe a cooling trend from 1 to 1800 CE that is robust against explicit tests for potential biases in the reconstructions. Between 801 and 1800 CE the surface cooling trend is qualitatively consistent with an independent synthesis of terrestrial temperature reconstructions, and with sea surface temperature simulated by an ensemble of climate model simulations using best estimates of past external radiative forcings. Single and cumulative forcing climate simulations suggest that the ocean surface cooling trend from 801-1800 CE is not primarily a response to orbital forcing but arises from high frequency of explosive volcanism. The results show that repeated clusters of volcanic eruptions can induce a net negative radiative forcing that results in a centennial and global-scale cooling trend via a decline in mixed-layer oceanic heat content.